Some automotive vehicles are equipped with starter alternators, that is to say an electric machine capable of starting the engine by using an electric source such as a battery or generating an electric current stored in the battery when the vehicle is driven by the engine. These machines can be used specifically in the vehicles equipped with a “STOP & START” function for which the engine stops as soon as the vehicle is at a stop, and starts again for example as soon as the driver accelerates again. More precisely, on these vehicles, the STOP & START function can be decomposed into three phases:                The initial start up phase of the engine—on a spark ignition engine (“gasoline” motor) or compression ignition engine (“diesel” motor). This initial phase is stilled called first start up phase.        The driving phase of the vehicle with the engine turning, generating zero acceleration from the vehicle (maintenance of the speed), positive acceleration or negative acceleration (i.e., deceleration). At the time of the deceleration situations, the STOP & START system can be designed to transform a part of the vehicle's kinetic energy into electrical energy, electrical energy that may be used by the set of consumer devices of the vehicle's onboard network or stored for example by means of the battery. In addition, the STOP & START system cuts the power of the engine as soon as the vehicle is at a stop, or even as soon as a stop is anticipated, for example as soon as the speed of the vehicle falls below a certain low speed threshold, which permits reducing the fuel consumption and therefore limiting the emission of pollutants.        The restart phase of the engine, ordered by the driver's command, marked for example by the depression of the accelerator pedal.        
The electrical machines of starter alternator type that permit the restarting of the motor have high energy requirements. At the time of the first start up, most electric consumers of the vehicle are normally at a stop. Such is not the case at the time of restarting of the engine, at which time equipment such as the air conditioner, the lighting system, the audiovisual system of the vehicle, etc. may be active, and remain so for the comfort and the safety of the vehicle's occupants.
However the electric machine dedicated to the restarting has a high current consumption that can generate high voltage drops on the whole of the onboard network and damage some components requiring electrical energy, resulting in a poor perception of the quality of the whole vehicle, with a failure rate that is sensed as random because the vehicle user does not necessarily associate the restarting of the vehicle motor with this fault (especially since the driver has not explicitly ordered the stopping of the motor).
In order to remedy this problem, different solutions have been proposed. The simplest, which is already implemented on commercial vehicles having the STOP-&-START function, consists in increasing the available electric power by coupling a second battery to the main battery. The main battery serves to supply the necessary electric power at the time of the first start up and restarting phases. At the time of this restarting, the electric functions sensitive to voltage variations are fed by a second battery—that is then the unique source of power for these functions at the time of this phase. Outside of this restarting phase (the duration of which is typically less than 1 sec), these sensitive functions are fed by the vehicle generator (for example the alternator).
This solution not only has a relatively high cost because of the additional battery—since the main battery must generally be replaced for example every two/three years but also because this additional battery cannot always be installed in the motor compartment and must be housed for example in the trunk, hence a need to reformulate the entire electrical architecture of the vehicle or even the entire structure of the vehicle. In addition, the mass of this additional battery and the electric bundles dedicated to it load down the vehicle goes contrary to meeting one of the objectives of this type of system, namely the reduction of fuel consumption and polluting emissions. Finally, this additional battery is generally dedicated to the consumer devices most sensitive to the voltage drops; therefore it does not necessarily constitute a complete solution to the problem of deterioration of the electric services, some equipment not supplied by this additional battery may see their performance decreased. This problem is aggravated by the multiplicity of accessories planned to be connected to an onboard network, such as for example the games consoles, video consoles, refrigerator, bottle warmer, etc., so that it is not always easy to ensure an appropriate sizing of the equipment, except by significantly over-dimensioning.
Another possibility consists in proposing to complete the battery by another type of energy storage means, such as for example a capacitor. The battery then supplies the necessary electric power to start the engine (first start up phase) of the vehicle and serves to supply the set of functions connected to it. The secondary means of storage is essentially used at the time of the restarting. But as the reference voltage of this secondary means of storage is variable, for example between one to three times the reference voltage value of the vehicle, this secondary means of storage cannot be connected to the onboard network if one foresees a DC/DC or analog type converter. There this solution still entails a significant surcharge, of the installation problems (the converter can require a volume of the order of 5 L for example) and increase the total mass of the vehicle (by the order for example of 5 to 10 kg).
More recently, some vehicles have been provided with a voltage maintenance device, still known under the (French) abbreviation DMT, mounted in series with the battery. The DMT is in fact an instantaneous power DC/DC compensator voltage converter, through which the under-voltage sensitive devices are fed at least at the time of the restarting phases, the DMT then takes its energy from the battery. This solution in fact poses similar problems to the solution consisting of splitting the battery, therefore making the electric architecture of the vehicle more complex, causing an increase of the vehicle mass, a loss of volume due to the footprint of the DMT and a reduction of the performance for the devices not supplied by this converter.
Furthermore, a supply system in electric energy is known from patent FR2853154 of consumer devices loaded onboard an automotive vehicle including the means of forming electric energy by a generator where the output is connected via an electrical network to a first means of electrical energy storage—such for example an electro-chemical battery—and to a first consumer device. A second consumer device is connected to the electric network via a means of forming a source of auxiliary energy, such as for example a capacitor, and the means of switching the operation which is controlled by means of a command to permit charging the second means of storage in series with the rest of the system and a connection of these with the second consumer device to ensure its supply at the time of its activation. Such a device permits increasing the voltage and power at the terminals of the second consumer device at the time of its activation, and therefore permits attenuating the drops in voltage associated with the start up of large energy consumer devices, without thereby over-dimensioning the means of energy generation and storage. Insofar as this device has the sole purpose of supplying instantaneous power to a function it does not afford any solutions at the time of the motor stop phases.